docs(cmd,github): clarify type assertion and parameter usage in review superseding

Address sonnet-review feedback on PR #106:

- Document that the type assertion in supersedeOldReviews is guaranteed to
  succeed given the caller's provider switch, with the !ok branch guarding
  against future refactors (comment 18889).
- Clarify that vcsURL is only used in the Gitea path for constructing
  review permalink URLs (comment 18890).
- Add note explaining why the page-limit warning in ListReviews only fires
  when the final page is full, confirming the logic is intentional
  (comment 18891).

github/reviews.go

@@ -140,6 +140,10 @@ func (c *Client) ListReviews(ctx context.Context, owner, repo string, number int
 		if len(reviews) < reviewsPerPage {
 			break
 		}
+		// NOTE: This warning only fires when the final page was full (the short-page
+		// break above did not trigger), meaning additional reviews likely exist beyond
+		// our page limit. The loop naturally exits after this iteration since page
+		// increments past maxReviewPages.
 		if page == maxReviewPages {
 			slog.Warn("ListReviews hit page limit; results may be truncated",
 				"owner", owner, "repo", repo, "pr", number,